Heretofore, a variety of diamond thin films and diamond-like carbon films have been known, and they are expected to be used as electrically insulating films, protective films, constituent members for electronic devices, and optical thin films. Some of them are already practically used as a speaker diaphragm, etc. As the method of producing such diamond thin films and diamond-like carbon films, various methods of synthesizing them from a vapor phase are suggested, and typical examples are:
(1) An ion beam deposition method (Japanese Patent Application Laid-Open No. 127298/1985); an ion beam spattering method (Japanese Patent Application Laid-Open No. 106513/1982); an RF spattering method (Japanese Patent Application Laid-Open No. 53255/1988); an RF glow discharge method (J. Fink et al., Solid State Comm. 47 (1983) 687); and a DC discharge method (Japanese Patent Application Laid-Open No. 145995/1985); and
(2) A microwave plasma CVD method (Japanese Patent Publication No. 3320/1986); a hot filament CVD method (Japanese Patent Application Laid-Open No. 91100/1983); and a hot plasma CVD method (Japanese Patent Application Laid-Open No. 158195/1987).
Any of the methods in (1) above generally gives a diamond-like carbon film high in amorphousness and having a smooth surface. However, any of the diamond-like carbon films obtained has problems given below.
The ion beam deposition method is a method wherein methane gas or the like is passed into an ion gun to form a carbon-containing ion beam, and the ion beam is struck to a substrate to thereby form a film. Although some of the films obtained by this method are detected as crystalline by the electron diffraction, generally the obtained films including a film called an i-c film are high in amorphousness and good in surface flatness. However, for these films, their characteristics are dependent on the properties of the amorphous parts as contained therein and they are still problematic with the points that the optical band gap is narrow, and the electrical resistivity is low, the film is far inferior to natural diamond.
The ion beam spattering method is a method wherein graphite is used as a target, and the target is spattered with a spattering gas in the form of an ion beam activated by an ion gun to form a film. The RF spattering method is a method wherein graphite is used as a target, and the target is spattered with a spattering gas activated with high-frequency electric energy to form a film. In any of these methods, although films having a low hydrogen content and high in hardness can be obtained, the films are poor in film properties including electric resistance and optical band gap, and because of this, they are not practically applicable.
The RF glow discharge method applies high-frequency electric energy to form a plasma, and the DC discharge method applies a direct current voltage to form a plasma, whereby a carbon-containing gas is decomposed to form a film.
By these methods, there may be obtained such films including a film called an a-C:H film which are amorphous and high in the H-content. For these films, there are advantages with respect to certain points that the optical band gap is wide and the electric resistance is high, but they are poor particularly in hardness and refractive index, and because of this, they are not practically applicable.
In addition to the above problems, the methods in (1) above are accompanied by the following other problems. That is, in any of these methods, the resulting films will be such that the number of carbon atoms per unit volume is generally small, even in the case of an a-C:H film not containing a long chain polymer structure of --CH.sub.2 --.sub.n', the density is undesirably small, and lacks stability particularly with respect to the etching resistance. In the methods mentioned in (1) above, the hydrogen content in the resulting film can be adjusted by controlling the substrate temperature (D. R. Mckenzie et al., Thin Solid Films 108 (1983) 247), or by controlling the substrate bias electric potential by the kinetic energy of the ion species involved in the film-formation (K. Yamamoto et al., Jpn. J. Appln. Phys. 27 (1988) 1415), or by controlling the self-bias electric potential, or by controlling with an acceleration voltage (C. Weissmantel et al., Thin Solid Films 96 (1982) 31). However, if the H-content in the film is lowered, many cases, without increasing diamond phases, result in increases in the number of SP.sub.2 hybrid carbon, the number of graphite structures, and the size of the conjugated system, leading to such problems that the optical band gap decreases, and the electric resistance lowers.
Further, in the case where the conventional diamond-like carbon film is annealed, it is gradually dehydrogenated to cause the film structure to change, whereby the film quality is lowered disadvantageously. In this case, there is another problem that graphite structures appear or increase in the resulting film thereby lowering not only its electric properties but also its stabilities such as etching resistance (B. Discheler et al., Solid State Comm. 48 (1983) 105, and Masahata et al., 2nd Diamond Symposium Advance Report (1987), page 7).
With respect to the methods mentioned in (2) above, it is generally possible for any of the methods to provide a diamond polycrystalline film that contains a slight amount of amorphous components. However, the films thus obtained are accompanied by problems as mentioned below.
That is, the microwave plasma CVD method is a method wherein a raw material gas is decomposed by a microwave plasma to cause the formation of a film. According to this method, because of electrodeless discharge, a film containing less impurities can be obtained, but because the film is formed with the substrate temperature kept as high as 800.degree. C. or over, the resulting film will become such that comprised diamond particles of about 10 .mu.m in size being scattered on a substrate or a so-called crystalline film comprised of such particles being continuously linked. The thus obtained film, if formed, lacks homogeneity and is poor in surface flatness, and because of this, it is not practically usable as a member.
The hot filament CVD method is a method wherein a carbon-containing gas is decomposed by way of a thermion releasing means. In comparison to the microwave plasma CVD method mentioned above, this method has an advantage that a film is possible to be formed on a large area substrate, but since the film formation is carried out under more or less around the same film forming conditions as those in the microwave CVD method, similarly to the film obtained by the microwave plasma CVD, the film thus obtained is not practically usable.
The hot plasma CVD method is a method wherein a film is formed by using a high-temperature plasma at a gas temperature of 1700 K or over. Since the film formation by this method is carried out under higher temperature and higher pressure conditions than those in the microwave plasma CVD method or in the hot filament method, a quality diamond of less crystal defects may be formed at a high speed, but the resulting film will become such that comprises diamond particles of larger sizes than those in the case of the microwave plasma CVD method or the hot filament CVD method, and such diamond particles being scattered on a substrate or being continuously linked, and because of this, it is not practically usable.
As another method of producing a diamond film or a diamond-like carbon film, a microwave plasma CVD method is suggested wherein a magnetic field is applied (see Japanese Patent Application Laid-Open No. 103098/1985 or Japanese Patent Application Laid-Open No. 36200/1986).
In Japanese Patent Application Laid-Open No. 103098/1985, although it is described that a quality diamond film may be obtained according to the method disclosed therein, the conditions are such that the substrate temperature is to be 700.degree. to 900.degree. C., and the film-forming pressure is to be 10.sup.-3 to 10.sup.-t Torr. However, if the film-forming pressure condition is made so low, and the substrate temperature is made so high, for example, the gas molecular density hardly reaches such an extent that allows a film to be formed, and therefore it is extremely difficult to stably obtain a desired diamond film as expected. Japanese Patent Application Laid-Open No. 36200/1986 describes that a diamond film may be formed at a high deposition rate. However, according to the method disclosed therein, the substrate is kept at a elevated temperature, and the pressure condition is made so high as 50 Torr. In this connection, a product comprised of a diamond should be obtained in accordance with this method, the product will be such that comprises diamond particles being scattered on a substrate or being continuously linked. Because of this, it is extremely difficult to stable obtain a practically applicable diamond film having a desired surface flatness.
Now, for an electrically insulating film or a protective film to be used in electronic devices, it is required for the surface of the film to be desirably even. In addition to this, particularly in the case where such film is used to protect a radiating body in electric or electronic devices which will be heated with passage of an electric current or light irradiation or in the case such film is used as a protective film to protect a constituent member in said devices from abrading upon contact with other member or certain materials, it is required for such film to have a desired heat stability, a desired etching resistance and a desired chemical stability represented by non-reactiveness with organometals, acids, alkalis, etc. However, there has not yet realized any desirable diamond-like carbon film which totally satisfies the above requirements, which has a desired surface flatness, and which is practically applicable.